Method for the production of corrugated tubes



METHOD FOR THE PRODUCTION OF CORRUGATED TUBES Original Filed March 5,1963 5 Sheets-Sheet l 5; Fig. [2a

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METHOD FOR THE PRODUCTION OF CORRUGATED TUBES Oct. 24, 1967 OriginalFiled March 5,

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METHOD FOR THE PRODUCTION OF CORRUGATED TUBES Original Filed March 5,1963 5 Sheets-Sheet 4 Fig. 7

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METHOD FOR THE PRODUCTION OF CORRUGATED TUBES Oct. 24, 1967 5Sheets-Sheet 5 Original Filed March 5, 1963 R O r N m N I United StatesPatent 6 Claims. ((31.264-99) ABSTRACT OF THE DISKILOSURE A method ofmaking a corrugated tube in which a tube of hardenable plastic materialis continuously extruded directly into a mold cavity havingcircumferentially corrugated walls while simultaneously advancing thewalls in the direction in which the tube is extruded, and whereinpressure fluid is fed into the interior of the extruded tube to applyinternal fluid pressure only to such portions of the extruded tube whichare located in the mold cavity to expand the tube during continuousadvancement of tube and walls into engagement with the corrugated wallsof the mold cavity to obtain thereby a corrugated tube.

This application is a division application of my copending application,Serial Number 263,680, filed March 5, 1963, now Patent No. 3,243,850,which in turn is a continuation-in-part application of my copendingapplication, Serial Number 861,806, filed December 24, 1959, and nowabandoned.

The present invention relates to a method of making flexible corrugatedtubes of plastic material, such as may be utilized to form protectiveinsulating sheaths for electric conductors and the like. Moreparticularly, the invention relates to a method for the production ofcircumferentially corrugated flexible tubing of hardenable plasticmaterial.

An important object of the invention is to provide a novel method ofcontinuously and rapidly producing circumferentially corrugated tubes ina simple and economical manner.

Another object of the present invention is to provide a method of theabove outlined characteristics which may be practiced in connection withthe manufacture of tubing having widely different diameters and Wallthicknesses.

A further object of the invention is to provide a method for themanufacture of corrugated tubing according to which the tube is formedin a continuous uninterrupted operation starting with a deformableplastic mass and ending with a selfsupporting flexible corrugated tubewhich is ready for storage, shipment or actual use.

With the above objects in view, the invention resides in the provisionof a method which comprises extruding a hardenable plastic material toform a continuous smooth-walled tube which is thereupon conveyed into amold cavity having corrugated mold walls, introducing a fluid pressuremedium into the interior of the smoothwalled tube, and permitting thepressure medium to act against the inner side of the tube whereby itdeforms the tube into engagement with the corrugated walls of the moldcavity. The wall of the extruded and still deformable tubular productthus assumes the shape of the mold cavity and is transformed into acorrugated tube, preferably while advancing in a direction away from thepoint of extrusion together with the molding means which latter definesthe aforementioned mold cavity. It is also preferred to cool thecorrugated product before removal from the mold cavity to insure thatthe tube may be withdrawn without deformation of its corrugated wallsand without adhering to the component parts of the corrugatingapparatus. According to a preferred method of the present invention, thetube while being extruded is also stretched in longitudinal and/orradial direction.

The novel features which are considered as characteristic for theinvention are set forth in particular in the appended claims. Theinvention itself, however, both as to its construction and its method ofoperation, together with additional objects and advantages thereof, willbe best understood from the following detailed description of a specificembodiment when read in connection with the accompanying drawings, inwhich:

FIG. 1 is composed of FIGS. la and 1b and is a schematic top plan viewof the corrugating apparatus with certain elements partly broken away;

FIG. 2 which is composed of FIGS. 2a and 2b, is a vertical section takenalong the line IIII of FIG. 1, as seen in the direction of the arrows;

FIG. 3 is a transverse vertical section taken on the line IIIIII of FIG.1, as seen in the direction of the arrows;

FIG. 4 is an enlarged front elevational and partly sectional viewshowing the mold cavity of a half matrix;

FIG. 5 is a top plan view of the half matrix;

FIG. 6 is an end elevational view of the half matrix;

FIG. 7 is a horizontal section through part of the apparatus illustratedat the right side of FIG. 2b, and showing at an enlarged scale anarrangement slightly modified from the arrangement shown in FIG. 2b;

FIG. 8 is a horizontal section through part of the apparatus and showingat an enlarged scale an arrangement slightly modified from that shown atthe left side of FIG. 2b; and

FIG- 9 is a schematic perspective view of a drive arrangement foradvancing the half matrixes through the two endless paths.

Referring now in greater detail to the illustrated embodiment, and firstto FIG. 1, there is shown a corrugating apparatus which comprises a baseplate 1 supporting the means defining a pair of endless, substantiallyoval paths 14, '15 for a pair of molding means in the form of two setsof half matrices 10, 11, repectively. Each path is shaped as a closedchannel and consists of two elongated parallel straight zones and a pairof arcuate end zones, one elongated straight zone of the path 14 beingadjacent to and merging with one elongated straight zone of the otherpath 15 to form a straight corrugating zone A.

As is shown in FIGS. 1 and 2, the corrugating apparatus is combined andcooperates with an extrusion press whose nozzle 2 is aligned with and islocated at the entry end of the straight corrugating zone A, andcoaXially receives a hollow mandrel 3. The members 2, 3 definetherebetween an annular extrusion passage 4 for the passage of a soft,hardenable plastic substance which is transformed in said passage into asmooth-walled elongated tubular body as it travels between the mandrel 3and the surrounding portion of the extrusion nozzle. The plasticmaterial is continuously delivered by a nonrepresented extruder whoseconstruction forms no part of my present invention.

The mandrel 3 is formed with a coaxial bore 5 for the passage of a fluidpressure medium, e.g., compressed air or another gas. The pressuremedium may be delivered by a compressor or the like of any known design.The lefthand end of the mandrel is internally threaded and meshes withthe externally threaded right-hand end of a coaxial connecting rod ormandrel 6. The connecting rod 6 has a central blind bore 7 whichcommunicates with the bore 5, as well as a series of preferablyuniformly spaced radial discharge ports or bores 8, e.g. four in number,for permitting the entry of fluid pressure medium from the bore 7 intothe interior of the smooth-walled deformable plastic tubewhich latter isextruded through the passage 4 and advances into and through the inletend of the corrugating zone A. Thus, the bores 5, 7 and 8 constitutepassage means for the introduction ofa compressed fluid into theextruded and still deformable tubular product. The fluid deforms thetube to the extent permitted by the corrugated walls of the moldcavities in mold sections formed by matching half matrices 10, 11advancing through the corrugation zone A. The nozzle 2 may be heated forinstance by electrical heating means embedded in the nozzle or heatedpressure fluid may be, introduced through the pass age means to assurethe necessary plasticity of the extruded tube.

The connecting rod or mandrel 6 extends through and beyond thecorrugating or deforming zone A and comprises, at a point distant fromthe extrusion nozzle 2, an enlarged portion or boss 9 whose diametercorresponds to the inner diameter to the corrugatedtube formed in thezone A; thus, the boss hinders the escape of pressure fluid from theinterior of the corrugated tubular product, e.g., an electric insulatingtube or the like.

FIGS. 7 and 8 show at an enlarged scale slightly modified forms of thearrangement shown in FIG. 2b. The arrangement shown in FIG. 7, differsfrom that shown in FIG. 2b,. in that the annular extrusion passage 4 hasat the free end of the extrusion nozzle 2, shown in FIG. 7 as the leftend of the extrusion nozzle, an outer diameter which is smaller than theminimum inner diameter d of the continuous elongated mold cavity formedby the complementary mold cavities of the half matrixes 10, and 11located in the corrugating zone A. It should be further noted that inthe arrangements shown in FIG. 2b as well as in FIG. 7 the free or leftend of the extrusion nozzle 2, extends into the right end, as viewed inFIGS. 2b and 7 of the elongated continuous mold cavity forming thecorrugating zone, or, in other words, the nozzle 2 extendsintocomplementary half-matrixes 10 and 11 which are fully abutting againsteach other along the abutment surface. 25 thereof.

The arrangement shown in FIGS. 7 and 8 differs also slightly from thearrangement shown in FIG. 2b, in that instead of forming an enlargedportion 9 integral with the connecting rod or mandrel 6, a separatesubstantially cylindrical member 9 is provided on the left end, asviewed in FIG. 8 on a connecting rod 6 which is screwed at the right endthereof, as viewed in FIG. 7 into an internally threaded bore of themandrel 3, and a rod 6" of a larger diameter than the rod 6' is screwedonto the left end, as

viewed in FIG. 8, of the connecting rod 6', holding thereby thecylindrical member 9 in a fixed position from the free end oftheextrusion nozzle2. The maximum diameter of the cylindrical member 9is one to three thousands of an inch smaller than the minimum diameterof the corrugated tube 38 produced in the apparatus of the presentinvention so that there is a very small clearance between thecylindrical member 9 and the corrugated tube produced. Despite thissmall clearance, escape of pressure fluid which is fed through into theinterior of the extruded tube through the passage 5 in the mandrel 6,the passage 7 in the right end, as viewed in FIG. 7, of the connectingrod 6' and the cross bores 8 communicating with the passage 7, ispositively prevented, since the pressure fluid will, in passing betweenthe cylindrical member 9' and the inner surface of the corrugated tube38 be successively compressed while passing through the small clearancegaps formed between the outer diameter of the cylindrical member 9andthe smaller inner diameter of the corrugated tube 38, and expanded inthe successive ring chambers 39 .formed by the corrugation of the tube.This successive compression and expansion of the pressure fluid willcreate such a turbulent flow so that any substantial escape of pressurefluid through the small gap between the cylindrical element 9' and thecorrugated tube 38 is substantially prevented.

The half matrices 10, 11 are of identical configuration, and one thereofis shown in three different views in FIGS. 4 to 6. It comprises anelongated mold cavity having circumferentially corrugated walls whosecorrugations are formed by alternately arranged parallel ribs 12 andgrooves 13 (FIG. 4), the ribs 12 forming grooves and the grooves 13causing the formation of ribs in the deformable tubular product extrudedthrough the annular passage 4. Each pair of halfzmatrices 10, 11 forms atubular mold section in the corrugatin-g zone A (FIG. 1), and theadjacent mold sections constitute an uninterrupted endless compositemold which may advance at the same speed at which the smooth-walled andstill deformable plastic tube is extruded through the annular passage ororifice 4.

As shown in FIGS. 2 and 3, the path 14 for the half matrices 10 isformed by an endless tubular assembly comprising a bottom plate 16,. apair of spaced upstanding walls 17, 18, and a cover plate or lid 19. Theinner wall 17 is continuous and forms an endless inner guide for theend-to-end disposed half matrices 10 which travel through the endlesspath 14 in clockwise direction, i.e. they reach the inlet end of thecorrugating zone A at a point adjacent to the extrusion nozzle 2 andthereupon advance to the left toward and beyond the dicharge end of thecorrugating zone to reenter said zone after having passed through theentire endless path 14.

The other endless path 15 for the half matrices 11 is formed by a bottomplate 16a, two spaced upstanding walls 17a, 18a, and a cover plate orlid 19a. As can be observed in FIG. 3, the bottom plates 16,16a maycon-- sist of a single piece of material which comprises a common web16b extending below the corrugating zone A.

The upstanding outer walls 18, 18a are broken away in and beyond thezone A so that the half matrices 10, 11

corrugating zone A are formed with cutouts or recesses 21, 21a (FIG. 3)covered by sheet metal or like plates 22, 22a to form two closedchannels for a coolant, e.g., water. A layer of packing material P isinserted between the adjacent faces of parts 17, 22 and 17a, 22a to forman airtight seal which prevents escape of coolant from the recesses 21,210, respectively. The recess, 21 receives coolant from a nonrepresentedhose connnected to an inlet nipple 23, and discharges spent coolantthrough a discharge nipple 24.- The other recess 21a communicates withan, inlet nipple 23a and a discharge nipple 24a. As may be observed inFIG. '1, the cooling medium flows in a direction counter to that inwhich the half matrices 10, 11 advance in the corrugating zone A. It isunderstood that the arrangement illustrated in FIGS. 7 and 8 may also beprovided with cooling means as described above, which for clarity sakeare not shown in FIGS.'7 and 8.

FIGS. 1 and 3 illustrate that the width of the corrugating zone A equalsthe combined width of straight zones of the paths 14 and 15 so that thematching half matrices may travel face-to-face in a direction to leftand away from the extrusion nozzle 2, their mold cavities receivingfirst the mandrel 3 and thereupon the guide member 6. While passingthrough the zone A, the half matrices 10, 11 form two-piece tubular moldsections by abutting against each other along their contact surfaces 25above and below the mold cavities (FIGS. 4 and 6) so as to define asingle circular mold cavity bounded by the corrugated walls formed bythe aforementioned ribs 12 and grooves 13. The mold cavity of eachmember 10 or 11 is bounded by two contact surfaces 25 which extend inthe longitudinal direction thereof. The upper and lower part of eachhalf matrix above and below the contact surfaces 25 is formed withinclined surfaces or facets 26, 27, 28 which form wedge-shaped spaces orrecesses when a pair of matching members 10, 11 advances through thecorrugating zone A. The wedges 20 (FIG. 1) are located at the level offacets 26-28 and, by sliding along the member 20, these facets guide thehalf matrices from the arcuate zones of paths 14, 15 into the straightcorrugating zone A. Similar wedge-shaped inserts 20a are provided at theleft-hand end of the zone A to insure smooth transition of half matricesback into the lefthand arcuate zones of endless paths 14 and 15. It willbe noted in FIG. 1 that the arcuate zones of paths 14, 15 are somewhatwider than the straight zones to prevent a pile-up of half matricestherein, i.e., the width of such arcuate zones exceeds the width ofindividual half matrices.

Each half matrix is formed at its underside with a toothed rack segment29 which extends the full length thereof (FIG. 4) and slides along thebottom plate 16, 16a or 16b while the half matrix advances in its path'14 or 15. The rack segments 29 of half matrices and 11 form twocomposite toothed racks each of which is straight in the corrugatingzone A as long as the contact faces 25 of the half matrices remain inabutment with each other. The toothed rack segments 29 act as a meansfor advancing the half matrices 10, 11 in their respective paths 14, bysuccessively engaging with a driven gear 30 which is located at a pointbelow the discharge end of the extrusion nozzle 2, i.e., at the entryend of the corrugating zone A, and whose axially parallel teeth 'meshwith the teeth of the rack segments 29 thereabove to advance the halfmatrices in a direction to the left, that is, away from the extrusionnozzle. The gear 30 is fixed to a horizontal shaft 31 which is driven byan electric or like motor, not shown. The plates 1 and 16b are formedwith a transverse cutout 32 through which the gear 30 extends into meshwith the toothed rack segments 29 of the matching half matrices 10, 11thereabove. The width of the gear 30 is so selected that its teethsimultaneously mesh with the rack segments 29 of aligned half matrices10, 11, i.e., the gear can mesh simultaneously with two parallel toothedrack segments 29. It will be readily understood that the shaft 31 maycarry two gears each of which will then mesh wit-h a single rack segment29.

A preferred drive arrangement is schematically shown in FIG. 9. In thisarrangement, the gear 30 which is adapted to mesh with the rack segments29 on the half matrices 10 and 11, not shown in FIG. 9, is fixedlymounted on a shaft 31 which carries on one end thereof a pulley 33fixedly connected thereto. The pulley 33 is driven over a V-belt 34 froma small pulley 35 fixed to the shaft of an electromotor 36. A speedregulator 37 is electrically connected to the motor 36 to vary the speedthereof, and to vary thereby the rotational speed of the gear 30 so thatdepending on the adjusted speed of the motor 36 the matrices or moldelement 10 and ,11 may be pushed with greater or smaller speed throughthe corrugating zone and along their respective endless path.

The corrugating assembly operates as follows:

The nonrepresented extruder delivers hardenable plastic material intothe discharge nozzle 2 which latter, together with the mandrel 3,transforms the material into a soft, deformable, smooth-walled tubularbody passing through the annular extrusion orifice 4 to be directlyextruded into the corrugating zone A in which the half matrices 10 and11 abut against each other to form an elongated mold cavity, to advanceabout the mandrel 3 and thereupon about the connecting rod 6. The gear30 is continuously driven by its shaft 31 and advances the matching halfmatrices 10, 11 through the corrugating zone in a direction away fromthe extrusion nozzle 2.

The arrangement is such that, as soon as one pair of members 10, 11 ismoved beyond the gear 30, the latter already meshes with the racksegments 20 of the next pair of half matrices 10, 11, and so forth, i.e.the matching pairs of half matrices 10, 11 are advanced in a continuous,uninterrupted train through and beyond the corrugating zone A. Therotational speed of the gear 30 may be selected in such a way that thehalf matrices 10, 11 advance at the speed at which the smooth-walledtubular body is extruded into the corrugating zone.

The smooth-walled tube enters the zone A in a still deformable state andis immediately subjected to the deforming action of a compressed fluidwhile advancing between the nozzle 2 and the boss 9 of the connectingrod 6. The compressed fluid enters the interior of the extruded tubularproduct through the channel defined by the bores 5, 7 and 8, and expandsthe tube by pressing its wall against the corrugated inner Walls of themold cavities formed by matching half matrices 10, 11. Thus, the tubeassumes the shape of the corrugated walls in the tubular mold cavitiesof advancing mold sections formed by the half matrices 10, 11, and istransformed into a circumferentially or helically corrugated body whichthen advances about the guide member 6, beyond the latters boss 9, andis evacuated through the discharge end of the corrugating zone at theleft-hand end of the apparatus (FIGS. 1 and 2).

When a drive arrangement as shown in FIG. 9 is used, it is possible toadjust the rotational speed of the gear 30 in such a way that the halfmatrices 10 and 11 will be advanced at a speed greater than that atwhich the smooth or tubular body is extruded into the corrugating zone.

Since the expanded and already corrugated tube 38 will be advanced atthe same speed as the half matrices 1i and 11, it is evident that byadvancing the half matrices '10 and 11 at a greater speed than that atwhich the tube is extruded into the mold cavity, the tube portionleaving the free end of the extrusion nozzle will be stretched inlongitudinal direction during the extrusion process. In

addition, if the extrusion passage 4 has at the free end of theextrusion nozzle a smaller diameter than the minimum diameter of themold mavity formed by the cor rugated walls of the half matrices 10 and11, as shown in FIG. 7, a considerable stretch in radial direction willalso be imparted on the tube material as it is being extruded into theelongated continuous mold cavity. This simultaneous stretching of theextruded tubular body in longitudinal and radial direction will arrangethe macromolecules of the plastic material in such directions which Willcorrespond to the actual stresses the finished tube will be subjectedduring use. The strength of the finished tube is thereby increased in anadvantageous manner over the strength of tubes in which such astretching of the material as it is being extruded is not carried out.

While FIG. 9 shows an electrical device for regulating speed of the halfmartices 10 and 11 could be regulated also in various different wayswhich will be obvious to any expert working in the fiel-d. For instance,the motor 36 could be formed as a gear motor with an adjustable geardrive or a stepless adjustable V-belt pulley arrangement well known inthe art may be used instead of the belt and pulley arrangement shown inFIG. 9.

While passing through the corrugating zone A, the tubular product iscooled by the liquid medium circulating in the recesses or channels 21,21a, and sets or hardens sufficiently to be removable from the machineas a selfsupporting flexible body. The matching half matrices open orseparate while passing along the wedge 20a and permit unobstructedadvance of the circumferentially or helically corrugated tubular producttoward and beyond the left-hand end of the guide member 6.

Without further analysis, the foregoing will so fully reveal the gist ofthe present invention that others can, by applying current knowledge,readily adapt itfor various applications without omitting features that,from the standpoint ,of prior art, fairly constitute essentialcharacteristics of the generic and specific aspects of thisinventionand, therefore, such adaptations should and are intended to becomprehended within the meaning and range of equivalence of thefollowing claims.

What is claimed as new and desired to be secured by Letters Patent is:

1. A method of making a corrugated tube comprising the steps ofcontinuously extruding a tube of hardenable plastic material directlyinto a mold cavity having circumferentially closed and corrugated wallswith the extruded end of said tube located within said circumferentiallyclosed and corrugated walls of said cavity while simultaneouslyadvancing the Walls in the direction in which the tube is extruded; andfeeding pressure fluid into the interior of the extruded tube to applyinternal fluid pressure only to such portions of the extruded tube whichare located in the mold cavity to expand the tube during continuousadvancement of tube and walls to expand the tube into engagement withthe corrugated walls of said mold cavity to obtain thereby a corrugatedtube.

2. A method of making a corrugated tube comprising the steps ofcontinuously extruding a tube of hardenable plastic material directlyinto a mold cavity having circumferentially closed and corrugated wallswith the extruded end of said tube located within said circumferen-,

tially closed and corrugated walls of said cavity while simultaneouslyadvancing the walls in the direction in which the tube is extruded; andfeeding heated pressure fluid into the interior of the extruded tube toapply internal fluid pressure only to such portions of the extruded tubewhich are located in the mold cavity to expand the tube duringcontinuous advancement of tube and Walls to expand the tube intoengagement with the corrugated walls of said mold cavity to obtainthereby a corrugated tube.

3. A method of making a corrugated tube comprising the steps ofcontinuously extruding a tube of hardenable plastic material directlyinto a mold cavity having circumferentially closed and corrugated wallswith the extruded end of said tube located within said circumferentiallyclosed and corrugated walls of said cavity while simultaneouslyadvancing the walls in the direction in which the tube is extruded;feeding pressure fluid into the interior of the extruded tube to applyinternal fluid pressure only to such portions of the extruded tube whichare located in the mold cavity to expand the tube during continuousadvancement of tube and walls toexpand the tube into engagement with thecorrugated walls of said mold cavity to obtain thereby a corrugatedtube; and cooling the Walls of the mold cavity to harden the plasticmaterial.

4. A method of making a corrugated tube comprising the steps ofcontinuously extrudin a tube ofhardenable plastic material directly intoa mold cavity having circumferentially closed and corrugated walls withthe extruded end of said tube located within said circumferentiallyclosed and corrugated walls of said cavity while simultaneouslyadvancing the Walls in the direction in which the tube is extruded at agreater speed than that at which the tube is being extruded into themOld cavity,

8. whereby the tube material entering the mold cavity will be stretchedin longitudinal direction so that by varying the speed at which the moldis advanced the wallthickness of the finished corrugated tube may bechanged independently of the wall thickness of the tube being extrudedinto the mold; and feeding pressure fluid into the interior of theextruded tube to apply internal fluid pressure only to such portions ofthe extruded tube which are located in the mold cavity to expand thetube during continuous advancement of tube and walls to expand the tubeinto engagement with the corrugated walls of said mold cavity to obtainthereby a corrugated tube.

plastic material with a predetermined outer diameter directly into amold cavity having eircumferentially closed and corrugated walls withthe extruded end of said tube located within said circumferentiallyclosed and corrugated walls of said cavity having a smallest innerdiameter which is larger than said predetermined diameter whilesimultaneously advancing the walls in the direction in which the tube isextruded; and feeding pressure fluid into the interior of the extrudedtube to apply internal fluid pressure only to such portions of theextruded tube which are located in the mold cavity to radially expandall portions of the tube during continuous advancement of tube and wallsand to expand the tube into engagement with the corrugated walls of saidmold cavity to obtain thereby a corrugated tube.

6. A methodof making a corrugated tube comprising the steps ofcontinuously extruding a tube of hardenable plastic material and of apredetermined outer diameter directly into a mold cavity havingcircumferentially closed and corrugated walls with the extruded end ofsaid tube located within said circumferentially closed and corrugatedwalls of said cavity of a smallest inner diameter which is larger thansaid predetermined diameter while simultaneously advancing the walls inthe direction in References Cited UNITED STATES PATENTS 2,663,90412/1953 Slaughter 26422 X 2,712,157 7/1955 Holte 18-19 X 2,760,2288/1956 Verges 1814 X 2,866,230 12/ 1958 Holte. 2,954,581 10/1960 Colombo18-5 FOREIGN PATENTS 61,855 5/1955 France.

ROBERT F. WHITE, Primary Examiner.

A. R. NOE, Assistant Examiner,

1. A METHOD OF MAKING A CORRUGATED TUBE COMPRISING THE STEPS OFCONTINUOUSLY EXTRUDING A TUBE OF HARDENABLE PLASTIC MATERIAL DIRECTLYINTO A MOLD CAVITY HAVING CIRCUMFERENTIALLY CLOSED AND CORRUGATED WALLSWITH THE EXTRUDED END OF SAID TUBE LOCATED WITHIN SAID CIRCUMFERENTIALLYCLOSED AND CORRUGATED WALLS OF SAID CAVITY WHILE SIMULTANEOUSLYADVANCING THE WALLS IN THE DIRECTION IN WHICH THE TUBE IS EXTRUDED; ANDFEEDING PRESSURE FLUID INTO THE INTERIOR OF THE EXTRUDED TUBE TO APPLYINTERNAL FLUID PRESSURE ONLY TO SUCH PORTIONS OF THE EXTRUDED TUBE WHICHARE LOCATED IN THE MOLD CAVITY TO EXPAND THE TUBE DURING CONTINUOUSADVANCEMENT OF TUBE AND WALLS TO EXPAND THE TUBE INTO ENGAGEMENT WITHTHE CORRUGATED WALLS OF SAID MOLD CAVITY TO OBTAIN THEREBY A CORRUGATEDTUBE.